1. Field of the Invention
The present invention relates to a process for forming tapered trenches in a dielectric material, in particular microtrenches for phase change memory cells having sublithographic dimensions.
2. Description of the Related Art
As is known, processes for manufacturing integrated circuits and devices often require etching trenches having predetermined profiles, either in semiconductor or dielectric materials. In particular, trenches with tapered walls are in many cases preferred to vertical trenches, since at least two advantages are provided. On the one hand, in fact, electrical field lines are less dense around tapered trenches than around vertical trenches, and, on the other hand, even very narrow tapered trenches are likely to be homogeneously filled, whereas gaps or air bubbles may remain inside vertical trenches.
Normally, a polymerizing plasma etching process is used to open trenches with tapered walls; such a process is particularly effective in etching dielectric materials, e.g., silicon nitride or silicon oxide. A mixture comprising an etchant gas and a polymerizing agent is supplied to the surface of a wafer, which is partially protected by a photoresist mask. Suitable etchant gases are generally based on fluorine compounds, such as CHF3, CH2F2, CF4 or SF6. A tapered profile is obtained because a polymeric passivating layer is deposited on sidewalls of the trenches while the etching is carried out. At the beginning, the whole wafer surface not protected by the mask and exposed to the plasma may be etched. As polymerization starts and the thickness of the polymer layer increases, the exposed area to be etched on the bottom of the trench is reduced. In practice, polymerizing plasma etch is based on a balance between the chemical etching of the exposed surfaces and the sidewall polymer deposition rate. When the polymer deposition rate prevails, a decreasing exposed surface is etched, so that the bottom width of the trench is reduced as its depth increases. Accordingly, the sidewalls are inclined (i.e., not vertical) and the trench has tapered profile.
However, known polymerizing plasma etching processes have some drawbacks. In the first place, of course, a polymerizing mixture is to be provided, further to an etchant agent, and a dedicated process step is required to remove the polymer passivation layer and to clean up the sidewalls of the trenches. Second, and more important, the balance between chemical etching and polymerization rate cannot be precisely controlled and errors may lead to useless trench profiles, especially in very thin layers having thickness of around 100 nm. For example, fluorine based chemical etching is very fast and tend to be isotropic against silicon nitride. As a consequence, when the process is unbalanced toward the side of chemical etching, U-shaped trenches are opened. On the contrary, etching process may be self-stopped, if the polymer deposition rate is too high, in this case, in fact, the polymer tends to deposit on the bottom of the trench as well, and prevents further etching.
Therefore, fluorine based polymerizing plasma etch is not suitable for making structures which require extremely accurate dimensional control, such as phase change memory (PCM) cells having a sublithographic dimension (i.e., a dimension that is lower than a minimum dimension obtainable through optical UV lithography).
As is known, phase change memory elements exploit the characteristics of materials which have the property of changing between two phases having distinct electrical characteristics. For example, these materials may change from an amorphous phase, which is disorderly, to a crystalline or polycrystalline phase, which is orderly, and the two phases are associated to considerably different resistivity.
At present, alloys of elements of group VI of the periodic table, such as Te or Se, referred to as chalcogenides or chalcogenic materials, can advantageously be used in phase change cells. The most promising chalcogenide alloy is formed by a combination of Ge, Sb and Te (Ge2Sb2Te5), which is currently widely used for storing information in overwritable disks. In chalcogenides, the resistivity varies by two or more magnitude orders when the material passes from the amorphous phase (more resistive) to the polycrystalline phase (more conductive) and vice versa.
In particular, in phase change memories, a thin film of chalcogenic material is employed as a programmable resistor, which can be electrically heated by a controlled current so as to be switched between a high and a low resistance condition. The state of the chalcogenic material may be read applying a sufficiently small voltage so as not to cause a sensible heating and measuring the current passing through it. Since the current is proportional to the conductance of the chalcogenic material, it is possible to discriminate between the two states.
PCM cells may be made by etching microtrenches through a silicon nitride layer of around 60-90 nm, by filling the microtrenches with the film of phase change material and by removing the film outside the microtrenches; the microtrenches preferably have bottom width of less than 100 nm. In this case, tapered profile is highly recommended, to favor filling, and the bottom width is critical because a suitable current has to flow through the microtrench base. It is clear that polymerizing plasma etch cannot ensure sufficient control of the microtrench profile and dimensions.